The exhaust of diesel engines, gasoline engines, and other internal combustion engines includes, for example, carbon monoxide (CO), unburned fuel comprised of hydrocarbons (HC), nitrogen oxides (NOX), particulate matter (PM), and other constituents. As devices for removing NOX, a reduction catalyst which continuously reduces the NOX which is contained in exhaust by the feed of a reducing agent to the exhaust purification catalyst or an NOX storage catalyst which stores NOX when the exhaust gas is lean in air-fuel ratio and releases the stored NOX when the exhaust gas is made rich in air-fuel ratio are known.
Japanese Patent Publication No. 11-81994 A1 discloses an internal combustion engine where a three-way catalyst is arranged at an upstream side and an NOX catalyst which reduces NOX by feeding of HC is arranged at the downstream side. It is disclosed that in this internal combustion engine, when the conditions for diagnosis of the catalyst stand, HC is fed in a pulse state to the three-way catalyst as a trace substance from an HC feed nozzle upstream of the three-way catalyst and the amount of adsorption of HC by the NOX catalyst (poisoned state) is estimated. Further, it is disclosed that the feed amount of HC which is fed as a reducing agent to the NOX catalyst is corrected in accordance with the amount of adsorption of HC by the NOX catalyst. It is disclosed that the greater the amount of adsorption of HC by the NOX catalyst, the smaller the HC feed amount should be set so as to prevent the amount of adsorption of HC by the NOX catalyst from exceeding a nonrecoverable poisoned limit value.
Japanese Patent Publication No. 9-4437 A1 discloses a nitrogen oxide removal system which is provided with a catalyst device which removes nitrogen oxides under an oxygen rich atmosphere and an HC increasing means for adding fuel as an HC increasing agent in the exhaust gas so as to increase the hydrocarbons in the exhaust gas. The control means of this removal system has a primary HC calculating unit which calculates the amount of increasing agent on a primary basis and a final HC calculating unit which corrects the primary calculated amount of the increasing agent and determines the final calculated amount. The primary HC calculating unit calculates the amount of the increasing agent on a primary basis from the state of the exhaust gas and the temperature of the catalyst bed. It is disclosed that the final HC calculating unit changes the amount of addition of the increasing agent based on the HC adsorption speed of the catalyst bed, the HC desorption speed, and the ratio of adsorption of hydrocarbons at the catalyst bed.
International Publication WO2011/114499 discloses an exhaust purification system of an internal combustion engine which arranges a hydrocarbon feed valve in an engine exhaust passage and which arranges an exhaust purification catalyst downstream of the hydrogen feed valve in the engine exhaust passage. This exhaust purification catalyst carries precious metal catalysts on an exhaust gas flow surface and is formed with basic exhaust gas flow surface parts around the precious metal catalysts. Further, it is disclosed to inject hydrocarbons, at the time of engine operation, from the hydrocarbon feed valve by a predetermined period of within 5 seconds and remove the NOX which is contained in the exhaust gas. With this exhaust purification system, it is possible to obtain a high NOX removal rate even if the exhaust purification catalyst becomes high in temperature.
Furthermore, International Publication No. WO2011/114501 discloses an exhaust purification system which injects hydrocarbons by the above predetermined period, wherein a first NOX removal method which maintains the exhaust gas which flows into the exhaust purification catalyst at the time of engine operation lean in air-fuel ratio while injects hydrocarbons from a hydrocarbon feed valve by a predetermined feed interval so as to remove the NOX which is contained in the exhaust gas and a second NOX removal method which switches the exhaust gas which flows into the exhaust purification catalyst from lean to rich in air-fuel ratio at intervals longer than a predetermined feed interval so as to remove the NOX are selectively used in accordance with the operating state of the internal combustion engine.